Compact near-eye displays often use plate-shaped (usually planar) waveguides for transmitting angular image information to users' eyes as virtual images from image sources located out of the user's line of sight. The image information is generally input near one end of the waveguides and is output near another end of the waveguides. The image information propagates along the waveguides as a plurality of angularly related beams that are internally reflected along the waveguide. Diffractive optics are often used for injecting the image information into the waveguides through a first range of incidence angles that are internally reflected by the waveguides as well as for ejecting the image information through a corresponding range of lower incidence angles for relaying or otherwise forming an exit pupil behind the waveguides in a position that can be aligned with the users' eyes. Often both the waveguides and the diffractive optics at the output end of the waveguides are at least partially transparent so that the user can also view the ambient environment through the waveguides, such as when the image information is not being conveyed by the waveguides or when the image information does not fill the entire field of view.
The waveguide displays are often limited to the use of monochromatic light in which the virtual images are formed in a single color. Conventional diffractive optics tend to diffract different wavelengths through different angles, creating chromatic aberrations. Multiple waveguides (e.g., stacked waveguides) or more complex diffractive optics can be used to mitigate these aberrations but the solutions tend to limit the number of wavelengths or the different angles through which the image information can be effectively transmitted.
The effective exit pupil size within which the virtual images can be seen in a prescribed position behind the waveguides (i.e., within a designed eyebox) is often overly limited because the diffracted light beams tend to spread apart upon leaving the planar waveguides. Variations in the diffraction efficiency of the output diffractive optics with position have been used to expand the exit pupils but these variations complicate the diffractive optics and are especially difficult to achieve with multi-chromatic light.
Some planar optic displays are also intended to support views of the ambient environment within the same eyebox. This requirement places additional burdens on the output diffractive optics to maintain some level of transmissivity.